As part of HSPA (High speed packet access) evolution, point to multi-point MBMS (multimedia broadcast multicast service) services will be offered on a dedicated downlink carrier in an entire coverage area termed as single frequency network (SFN). This new SFN feature is in addition to the release 6 type of MBMS transmission mechanism, which takes place on the existing carrier.
This section describes various technological aspects and principles of MBMS transmission in UTRAN (UMTS Terrestrial Radio Access Network).
Release 6 MBMS Transmission
Release 6 supports the following two MBMS transmission modes, see 3GPP TS 25.346, “Introduction of the Multimedia Broadcast/Multicast Service (MBMS) in the Radio Access Network (RAN); Stage 2”. 3GPP TS 25.331, “Radio Resource Control Protocol Specifications”:                Point to point MBMS transmission        Point to multipoint MBMS transmission        
In both the above scenarios the MBMS services are shared with other conventional services such as speech, data etc., on the same carrier.
The point to point MBMS transmission can take place on DCH (Dedicated channel) or HS-DSCH channel (High Speed Downlink Shared Channel). On the other hand, point to multipoint MBMS services are sent on S-CCPCH (Secondary common control physical channel) from multiple cells. In the case of MBMS transmission on S-CCPCH, the UE can combine the MBMS transmissions from more than one cell. Either selective or soft combining is used.
Dedicated MBMS Transmission Via SFN
As part of HSPA evolution there is likely to be a separate dedicated carrier for MBMS transmission. In this scenario MBMS services will be transmitted on one carrier dedicated to MBMS services only. This is only a point to multipoint transmission scenario. This scenario will be characterized by a single frequency network (SFN), enabling SFN combining (i.e. combining in the air). This means the same service should be sent on the same physical resource in all the multi-cells, which are SFN combined. Similarly the MBMS control channel should also be SFN combined, i.e. it must also share the same physical resources in all combined cells. Secondly all the resource blocks containing MBMS shall use the common scrambling code in all the mixed cells within the SFN area. The main advantage of this approach is SFN combining gain and the users can move around the SFN area transparently while receiving the MBMS data.
SFN based MBMS transmission is also supported in E-UTRAN, see 3GPP TR 25.913, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN).
Cell Reselection and System Information Reading
In WCDMA, when a UE in idle mode moves in a coverage area, it performs cell reselection. In other words the UE camps on the best cell and reads the system information, which is sent on the BCH channel, mapped on P-CCPCH (Primary common control physical channel), see 3GPP TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD). In WCDMA the BCH is transmitted continuously, i.e. code multiplexed with other channels. The transmission bit rate (coded data rate) is around 30 kb/s. The UE requires up to 1 second for reading the entire system information broadcast on BCH. The UE in idle mode may also be required to read the system information in between the cell reselection in case there is any change in the system information as indicated by paging indication.
The main problem is the loss of MBMS data due to BCH reading in the case that the UE is receiving MBMS on the SFN network.
A problem with existing solutions is that in principle the SFN transmission could be interrupted to allow the UE to read other channels on the non-dedicated MBMS cells, especially the system information. Short interruptions to read paging and to make neighbour cell measurements are acceptable from the MBMS reception performance point of view. On the other hand, longer interruptions in the order of seconds (one or more) will severely degrade the MBMS reception. In the current solution, due to continuous BCH transmission, the UE in order to read BCH after cell reselection (reselection of non-dedicated MBMS cell) will be required to interrupt the MBMS reception for at least one second.
One solution to this problem is to have a dual receiver at the UE. This means the UE can receive MBMS data on one receiver and read BCH and other channels on another receiver. However, dual receivers add more complexity and increase cost. A dual receiver leads to more severe in-band inter-modulation products due to circuit non-linearity. It also generates more severe self-mixing effects due to local oscillator leakage via substrate or other coupling effects. In a homodyne receiver, this results in worse DC-carrier performance. In general, a two synthesizer solution could also give worse spurious emissions. In terms of implementation complexity the interface between RF (Radio Frequency) and BB (baseband) must be redesigned to support the additional reception. Similarly, support for baseband processing of an extra receiver chain is also needed.